[Show abstract][Hide abstract] ABSTRACT: An electrostatic ion lens to spatially orient parent molecules and to image the angular distribution of photofragments is presented. Photodissociation of laboratory-oriented molecules makes it possible to study the dynamics of the dissociation process in more detail compared to photodissociation of nonoriented molecules. Using the velocity map imaging technique in combination with the slice imaging technique, the spatial recoil distribution of the photofragments can be measured with high resolution and without symmetry restrictions. Insertion of orientation electrodes between the repeller and the extractor of a velocity mapping electrostatic lens severely distorts the ion trajectories. The position where the ions are focused by the lens, the focal length, can be very different in the directions parallel and perpendicular to the inserted orientation electrodes. The focal length depends on the exact dimensions and positions of the electrodes of the ion lens. As this dependence is different in both directions, this dependence can be used to correct for the distorted ion trajectories. We discuss the design of an electrostatic ion lens, which is able to orient parent molecules and map the velocity of the photofragments. We report sliced images of photofragments from photolysis of spatially oriented CD3I molecules to demonstrate the experimental combination of molecular orientation and velocity map slice imaging with good resolution.
Full-text · Article · Dec 2005 · Review of Scientific Instruments
[Show abstract][Hide abstract] ABSTRACT: We present single-photon spectroscopy in molecular hydrogen starting from the metastable c3Piu- state to a number of triplet nd-Rydberg states (v = 0 - 4, n = 12 - 20). Using fast beam spectroscopy both the autoionization channel and the predissociation channel are quantified, field free, as well as with small electric fields. Coupling with the i3Pig state is assumed to be responsible for field-free predissociation of the v = 0 Rydberg levels. The stronger observed predissociation channel of the v = 1 Rydberg levels is due to the nonadiabatic interaction with the h3Sigmag+ state in combination with l mixing due to an external electric field. No direct evidence is found for possible electric field induced predissociation of the gerade Rydberg states by low lying ungerade states. The competition between autoionization and predissociation is discussed in terms of possible consequences for dissociative recombination involving low energy electron collisions with the H2+ molecular ion.
Preview · Article · Sep 2004 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: Metastable states of the tetraoxygen (O4) molecule produced in a pulse electric discharge of molecular oxygen were discussed. It was found that a strong signal cation and photoelectron signals arised from the ionization of a neutral species instead of the photodissociation of a larger ion. It was observed that ground state iron atoms were present in the discharge beam. It was also found that only less than half of the spectra peaks for cations and photoelectrons observed were present in the Fe NIST emission lines table.
Full-text · Article · May 2004 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: We present an experiment on the photodestruction of the NO2− anion at 266 nm. We have quantified the competition between photodetachment and photodissociation and have identified the nature of the photodissociation process from the photofragment angular distribution. This study involves a novel technique; time resolved multicoincident detection photofragment spectroscopy. A three-dimensional double exposure CCD camera is employed. The system provides the position (x,y) and the relative arrival time (t) of all fragments. In the case of photodissociation events, NO and O− fragments are detected for each event. The detection of multiple events per laser shot is made possible by using center-of-mass selection.
No preview · Article · May 2004 · Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: Dissociative Recombination (DR) has been a subject of research for more than 50 years.1 In a DR event, a molecular ion captures an electron forming an intermediate electronically excited neutral molecule that rapidly dissociates into fragments: AB++e(E)→AB**→A+B*+KER. The symbol ‘KER’ stands for the kinetic energy released to the fragments during the reaction and E is the electron’s energy. Among the numerous species studied so far, the molecular ions H2+, H3+, and HeH+ and their isotopomers have been studied in most detail. DR of molecular hydrogen serves as the prototypical DR reaction. In spite of its apparent simplicity, being a one-electron system, DR of H2+ is theoretically challenging. Although theory is making progress,2,3,4 theory cannot yet be used as an a priori reliable standard. In H2 the direct process, as formulated by Bates, is prominent. The lowest neutral doubly excited state, Q11Σg+, crosses the ionic state close to the outer turning point of the ground vibrational level. The lowest triplet doubly excited state, Q13Πg, does not have such a favorable crossing.5 As a consequence, theory generally assumes that the direct process is very slow for triplet collisions. In systems in which the direct process is impossible, such as HeH+, an alternative mechanism has been formulated in which the ion plus free electron is coupled with bound Rydberg states through the radial part of the kinetic energy operator.6,7 In HeH+, this mechanism explains a substantial DR cross section at low electron collision energies.